The present invention relates to a helium gas-refrigerating and liquefying apparatus which will be abbreviated occasionally as "apparatus" hereinafter.
Recently, accompanying the development of superconductivity technology, demand for liquid helium has increased rapidly. A helium gas-refrigerating and liquefying apparatus which produces liquid helium is, usually, composed of a compressor, heat exchangers and an expansion machine. In order to improve reliability and efficiency of such apparatus of large size, many researches and developments have been made, especially in regard to heat exchangers and expansion machines. As a result, many technical problems of heat exchangers and expansion machines have been solved. However, large size compressors have not been developed sufficiently and still have technical problems.
A prior art apparatus for generating cold of a temperature range of 1.8.degree.-20.degree. K. is shown in the attached FIG. 1. When using the apparatus, helium gas is compressed by a helium compressor 1 to a high pressure of about 10-15 atm, and the high pressure helium gas is transported to a heat exchanger 2 wherein it is heat exchanged with low temperature return helium gas coming from an expansion turbine 5 through a heat exchanger 3 and from a Joule-Thomson valve 6 through heat exchangers 4 and 3 thereby to decrease its temperature. A portion of the helium gas exited from the heat exchanger 2 is distributed to the expansion turbine 5 to do work therein and decrease its temperature to become a portion of the aforementioned low temperature return helium gas. The rest of the high pressure helium gas from the heat exchanger 2 is passed through heat exchangers 3 and 4 to further decrease its temperature, and subsequently transported to the Joule-Thomson valve 6 wherein it is adiabatically freely expanded to further decrease its temperature. As a result of the adiabatic free expansion and decrease of temperature, a portion of the helium gas is liquefied in the Joule-Thomson valve 6, which is in turn transported as a charge to a superconducting magnet or the like device 7 to cool the same.
In the aforementioned helium compressor, heretofore use has been made of a piston type compressor or a screw type compressor. However, piston type compressors have low reliability over a long period of operation, though they have good properties such as high isothermal efficiency. In contrast, screw type compressors have low isothermal efficiency, through they have good reliability over a long period of operation. In addition, both the piston type compressors and the screw type compressors have a drawback that their sizes become unavoidably large.
Instead of using a piston type compressor or a screw type compressor, adoption of a turbo type compressor having superior characteristics from the view points of size, reliability and properties as compared with the piston type compressors and the screw type compressors could be considered for rapidly improving the reliability and the properties of the large size apparatus and for minimizing the size thereof. However, helium gas has a low molecular weight of 4 and a high mean molecular velocity at an ambient temperature, so that it can not be compressed efficiently to a high pressure of, e.g., about 10 atm in a turbo type compressor. Therefore, hitherto, a helium gas-refrigerating and liquefying apparatus using a high pressure turbo type compressor was not practiced as far as the inventors know.